This invention relates to an electric fuse, and more particularly to such a fuse with indicating means which can be non-destructively tested.
Current limiting power fuses generally have a cylindrical housing of insulating material which is closed at each end by a metal terminal cap. A main fusible element(s), usually a silver ribbon, extends inside the housing between the terminal caps. The space around the fuse element is filled with silica sand. When current through the fuse exceeds the rating of the fuse for a sufficient time, the element melts, or fuses at one or more points, causing the formation of an arc. The arc progressively melts the sand and forms channels of fulgurite in it. The relatively high resistance fulgurite eventually suppresses any significant flow of current through the fuse.
The melting of the main fusible element is generally silient, and hence, not noticeable. Thus, it is common to provide indicating means by which it can be readily determined if the main fusible element has melted. Such indicating means generally comprises a high resistance auxiliary fusible element, of a material such as the one available under the trademark Nichrome, disposed between the terminal caps, and in parallel electrical relation with the main fusible element. A portion of the auxiliary element is placed under tension and connected to an indicating means assembly which includes a movable indicator. In operation of such a fuse, when the main element melts, current is diverted through the auxiliary element. This results in the melting of the auxiliary element and movement of the indicator. Exemplary movable indicator mechanisms are described in U.S. Pat. No. 3,895,338 issued July 15, 1975 to Gray et al, and assigned to the assignee of the present application, and in U.S. Pat. No. 2,797,279, issued June 25, 1957 to Brandt et al.
Although such fuses are successful for many applications, there are some problems associated with their use and assembly. One such problem is that, after assembly of the fuse and indicating means, there is no means for nondestructively determining if the indicating means has been properly assembled, and hence, in operable condition. More particularly, the auxiliary fusible element is permanently attached in parallel with the main fusible element. The main fusible element is of much lower resistance than the auxiliary fusible element so that it is difficult to determine if the auxiliary fusible element is operable by measuring the fuse resistance between the terminal caps. This is due to the fact that the resistance of the auxiliary fusible element adds very little to the total fuse resistance. This causes it to be virtually impossible to detect a resistance difference which would indicate whether or not the auxiliary fusible element is in operable condition. One proposed solution to this problem is to X-ray the fuse so as to determine the condition of the auxiliary fusible element. However, it has been found that the generally heavy copper ferrules and end caps mask out the area at the ends of the fuse so that the auxiliary element cannot be observed in those areas. Also, this procedure requires the use of sophisticated equipment not generally available to common users of such fuses.
Accordingly, it is a general object of my invention to provide an electric fuse assembly with indicating means in which the operability of the indicating means can be reliably determined.
It is another object of my invention to provide such an electric fuse in which the indicating means includes an auxiliary fusible element whose resistance can be nondestructively and independently tested.